This invention relates to a method for igniting a lean mixture charge such as a gasoline or other fuel in a lean mixture charge type internal combustion engine, and a device therefor, to minimize to a greater extent the concentration of detrimental constituents in exhaust gases from the engine.
It is generally known that production of detrimental constituents such as CO, HC, NOX and so forth, in exhaust gases from an internal combustion engine is largely dependent upon the air-fuel ratio of the charge mixture. As a means of igniting a lean charge whose total air-fuel ratio is lower than a theoretical air-fuel ratio, it has been proposed that the charge be stratified under the influence of a corona discharge in a combustion chamber. The corona discharge ionizes fuel particles to collect a rich mixture in the vicinity of a spark plug. The apparatus for producing the corona discharge includes an electrode disposed in the vicinity of a spark gap in either a main combustion chamber or a subsidiary combustion chamber. A high voltage is impressed across the electrode and the combustion chamber wall, thereby producing a corona discharge between the tip of electrode and the inner wall of the combustion chamber.
The device described above is somewhat unsatisfactory in that electrically charged fuel particles are attracted to the wall of the combustion chamber in the vicinity of the electrode and cooled. Due to a well known quenching phenomenon, there is incomplete combustion of the relatively cool fuel particles attracted to the combustion chamber wall. This results in the exhausting of an undesirably large quantity of hydrocarbons.
An ignition method and a device for use in an internal combustion engine has been disclosed in Japanese patent applications Nos. 48-78009 (Public Disclosure No. 51-7309), 50-89549 and 50-127841, as well as in U.S. patent application No. 491,452 filed June 30, 1975 and entitled System and Device for the Ignition of an Internal Combustion Engine Using A Lean Air-Fuel Mixture. According to this ignition system, a relatively large D.C. voltage is impressed on a center electrode of a spark plug disposed within an auxiliary combustion chamber to cause a corona discharge. Fuel particles contained in an air-fuel mixture are electrostatically collected and atomized in the vicinity of the spark plug due to an electric field produced across the tip of the center electrode and a peripheral electrode surrounding the tip of said center electrode. This promotes stratification of the air-fuel mixture with a relatively rich mixture adjacent to the spark plug. The burning of the stratified charge is relatively efficient since the fuel particles are electrostatically attracted to the electrodes rather than the combustion chamber wall so that there is a minimal amount of quenching.
The advantages of the ignition method and apparatus of the aforesaid Japanese and United States patent applications has been proven by actual tests. The tests were carried out in a light van loaded with a four-cycle 1500cc engine. The engine had four-cylinders with wedge type cylinder heads. The van body weighed 2250 lbs. No antipollution devices were utilized other than the ignition method and apparatus disclosed in the aforesaid Japanese and United States patent applications.
In order to utilize the improved ignition method and device disclosed in the aforesaid Japanese and United States patent, internally threaded spark plug holes in the cylinder head were increased in diameter to 20mm. Members defining auxiliary combustion chambers having a spark plug therein were attached to the cylinder head at the enlarged spark plug holes. The spark plugs connected with the auxiliary combustion chambers had corona discharge center electrodes, cylindrical peripheral electrodes which surrounded the center electrodes, and spark ignition electrodes.
The tests were carried out in accordance with LA-4 modes set forth in EPA specification of United States and the test results are given in Table 1. For reference, there are shown the limits for LA-4 modes set forth in USA regulation and those set forth in California regulation which were in force in 1951, as well as the numerical values used in the light van prior to modification of the engine i.e., prior to installation of the improved ignition method and device disclosed in the aforesaid Japanese and United States patent applications.
TABLE 1 ______________________________________ Improved Engine In Which the Invent- Detri- ion of the Afore- mental US Califor- Prior said patent appli- Consti- Limits nia Limits Art cations was tuents * * Engine practiced ______________________________________ CO 15 9 (5.0) 12.7 7.8 HC 1.5 0.9(0.6) 1.04 0.3 Nox 3.1 2.0(1.5) 1.73 1.69 ______________________________________ Fuel The numeri- 23.1 27.0 consump- cal values tion in parenthe- mile/ sis are gallon target values for 1981 ______________________________________ *As of September, 1975 Remarks: (1) The numerical values of gases are expressed in mile/gallon. (2) In the case of an improved type engine, a mixture charge used was rendered leaner by adjusting a carburetor.
Table 1 clearly shows that, by adopting the ignition method and device according to the invention of the aforesaid Japanese and United States patent applications to an engine in which no antipollution measures are taken, the quantity of detrimental exhaust gases from the engine and fuel consumption are greatly reduced. However, the ignition method and device of the aforesaid Japanese and United States patent applications is not as effective on engines having a semi-spherical shaped combustion chamber. In addition, it should be noted that the quantity of exhaust gases from the improved engine is too large to meet the target values of United States EPA regulation for 1981.